This invention relates to a novel method of bonding lignocellulosic materials and to a new and useful thermosetting binder employed in bonding same.
The most commonly used adhesive binders for bonding lignocellulosic materials into, for example, particleboard and waferboard are thermosetting urea-formaldehyde (UF) and phenol-formaldehyde (PF) resins; with the more expensive phenol-formaldehyde thermosetting resin being used for exterior grade products. As the UF and PF resins are petroleum based synthetic resins, from an economic standpoint, the reconstituted wood products industry has attempted to develop suitable interior and exterior adhesive substitutes from wood waste in order to solve environmental disposal problems and to reduce the industry's dependence upon the uncertain supplies and cost of petroleum based resins.
There have been numerous proposals for utilizing spent sulfite liquor (SSL), the by-product of the sulfite pulping process, as a adhesive binder for particleboard and waferboard but none of these proposals has to date, proven entirely satisfactory.
In context of employing ammonium lignosulfonate which can also be described as ammonium based spent sulfite liquor (NH.sub.4 SSL), various attempts have been made to cross-link the PF with the NH.sub.4 SSL. For example, Herschler in U.S. Pat. No. 2,786,008 issued Mar. 19, 1953 discloses the use of a binder consisting of an aqueous solution of phenol-formaldehyde with ammonium based sulfite liquor. Herschler recognized that ammonium based spent sulfite liquor which is acidic in character was not miscible with phenol-formaldehyde, since upon mixing, the resin would precipitate out. This was overcome by making the PF "acid tolerant" by first condensing the resin to the late A or early B stage and thereafter, adding an agent, such as oxalic acid, to remove the sodium oxalate so formed by means of filtration. In addition to being laborious and expensive, the adhesive obtained has been found to be slow curing.
Wiegand et al in Canadian Pat. No. 735,389 issued May 31, 1963 discloses an alkaline process in which water soluble alkaline kraft or lignosulfonate is mixed with water soluble high molecular weight PF. The water soluble mixture at high solids content may be used as a tackifying agent and adhesive for particleboard and fibreboard. The unique reactivity of NH.sub.4 SSL towards PF was, however, not recognized as the best results were obtained with NaSSL and kraft lignin at high pH. The mixture as disclosed is slow curing at a temperature of 230.degree. C. which was utilized to cure particleboard (normally cured 170.degree. C.) with a product of low strength being obtained even with a large resin content. In addition, the SSL-PF could not be used as an additive where the SSL and PF were found incompatible. The compatibility, water solubility and mixing ratio considerations as taught by Wiegand et al are not considered controlling in accordance with our invention.
Shen et al in U.S. Pat. No. 4,265,846 issued May 5, 1981 recognized that chemical modification to the spent sulfite liquor is not desirable as it necessarily contributes to increased resin cost. Crude or low molecular weight ammonium based spent sulfite liquor was thus used alone as the thermosetting binder and hot pressed at 170.degree. C. or higher for a time sufficient to decompose the NH.sub.4 SSL into lignosulfonic acid and ammonia gas, whereafter the acid condensed and polymerized in the presence of liqnocellulosic materials to produce an exterior grade wood composite product. However, this decomposition process is slow, as approximately 14 minutes passing at 210.degree. C. is required to produce an exterior grade 11.1 mm waferboard and approximately 8 minutes required when low molecular weight NH.sub.4 SSL is utilized as the binder. Due to the long press cycles when compared with PF binders that have a press cycle of 3 to 4 minutes, it is unlikely this technique will receive signifigant acceptance by the industry. Moreover, NH.sub.4 SSL as a resin suffers the disadvantage or resin sticking to the metal caul plates used during heat application and pressing. It has also been observed that if the carbohydrates are removed, NH.sub.4 SSL looses its adhesive properties.